Note: Descriptions are shown in the official language in which they were submitted.
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FURNACE SUPERHEATER AND A FLU117IZED BED REACTOR PROVIDED
WITIi A FURNACE SUPFR~TRA~g
' The present invention relates to a furnace superheater having a number of
tube
elements mounted to each other in such a way that they form a planar surface.
The present invention also relates to a circulating fluidized bed reactor
provided
with a furnace superheater, comprising a reactor chamber, a solids separator
connected thereto and a return duct, which connects the solids separator to
the
reaction chamber. Evaporator surfaces and a steam superheater mounted in the
furnace of the circulating fluidized bed reactor are also provided.
Solid material, such as fuel, inert bed material andlor limestone is fluidized
in the
circulating fluidized bed reactor with gas supplied through gas feed means
located
in the lower part of the reactor. The fluidization velocity in circulating
fluidized
bed reactors is in normal conditions so high that a substantial portion of the
bed
material is entrained with the gas and discharged out of the reactor. It is
especially
a characteristic feature of the circulating fluidized bed that the amount of
solid
material being entrained with gas is so high that the operation of the bed
could not
be maintained without recirculation (or addition) of solid material.
In a circulating fluidized bed reactor significant amounts of solid material
is
discharged with the gases out of the reactor through the upper part thereof.
In
certain areas of the reactor both upward and downward flow take place. The
absolute material flow varies both radially and axially in the reactor, and
the
downward flow is at its maximum close to the outer walls. When the particle
density increases towards the lower part of the reactor.chamber, also the
particle
accumulation along the outer walls increases downwards. Even a slight change
in
the flow direction of the particles causes erosion. Thus a circulating
fluidized bed
reactor provides very demanding conditions for different structures.
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la
It is known from EP 0 011 102 to mechanically tie together
tubes of a superheater platen assembly. The tie members used
are essentially L-shaped in configuration and affixed by
wElding to the tubes.
It is also known from US 2,809,616 a superheating unit with
pendent superheater platens, in which the tubes of each platen
are connected by rows of clevises including slidably
interfitting parts, such as T-slotted clevises. This
arrangemenet permits relative movement between the separate
tubes. In other parts the sleeves may be welded to the tubes,
adjacent tube sleeves being connected by pins extending
therethrough.
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Due to the erosive conditions of the circulating fluidized bed reactor, it has
been
suggested that tube elements were used, which when connected to each other
form
a smooth outer surface. This kind of a structure has been suggested to be
used,
e. g. in a furnace superheater in power plant boilers based on circulating
fluidized
bed technique, whereby the superheater may be located directly in the furnace
of
the circulating fluidized bed reactor. Such tube elements are attached to each
other
by welding. Welding includes always a number of time-consuming operational
steps, such as preheating, pre-welding, welding in tube direction on two sides
of
the tube, correction of deformations caused by stresses and inspection of the
welding. A superheater panel formed by this welding method is therefore almost
inevitably assembled to form one single structure element. Welding is made on
both sides, whereafter both surfaces are machined to give an as smooth as
possible
surface. This is recommended, as the conditions in a circulating fluidized bed
reactor, even in the upper part thereof are so erosive that sufficient
reliability
cannot otherwise be guaranteed.
Forming superheater surfaces from parallel round tubes without welding them is
appreciated from US patent 5,012,767. There a3jacent tubes are attached with
each
other by a two sleeve-combination. The tubes run through the sleeves and the
sleeves are ~veided to each other. This round, sleeve-like arrangement is very
liable
to erosion and the attaching sleeves also form a significant, easily eroding
point of
discontinuity. The same patent publication also discloses a possibility of
welding
separate attaching means between the tubes to connect them to each other. Even
this arrangement provides a heat exchanger very liable to erosion and this
kind of
arrangement also increases the distance between the tubes, whereby even less
parallel tubes can be fitted in the same space.
An object of the present invention is to eliminate or minimize the need of
welding
in connection with the manufacture of in furnace supe~rheater tubes, whereby
the
manufacture time of a panel structure significantly diminishes. At the same
time,
operational welding steps, such as preheating, pre-welding, welding in tube
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direction on both sides of the tube, levelling of deformations caused by
stresses
and inspection of the welding are eliminated or minimized.
Another object of the present invention is also to provide a circulating
fluidized
bed reactor, in which furnace superheaters are formed as planar surfaces to
correspond to the prevailing conditions in a very simple manner.
Further, an object of the present invention is to provide a circulating
fluidized bed
reactor, in which furnace superheaters are formed as wear-resistant
structures.
A purpose of the present invention is to eliminate or minimize disadvantages
of
lrnown technique and to provide a completely novel construction. The invention
is
based on the concept that a furnace superheater is formed substantially
without
welding.
More accurately, a characterizing feature of a furnace superheater
according to the present invention is mainly that the tube elements are
connected
to each other mechanically.
Also a characterizing feature of a circulating fluidized bed reactor according
to the
present invention, said reactor being provided with a furnace superheater,
comprising a reaction chamber, a solids separator attached thereto and a
return
duct, connecting the solids separator to the reaction chamber, evaporator
surfaces
and a steam superheater arranged in the furnace of the circulating fluidized
bed
reactor, is mainly that said superheater comprises a planar tube panel formed
of a
number of superheater tube elements mechanically attached to each other.
According to a first embodiment of the invention a furnace superheater, which
is
especially suitable for a circulating fluidized bed reactor, is formed of
superheater
tubes of a rectangular cross-section, which are attached to each other by
means of
grooves or the like. In this connection, the term "rectangular" means a form
having at least two substantially parallel sides, which are at least partially
on top
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of each other. A furnace superheater in accordance with the invention is thus
formed by a number of separate superheater tubes attached to each other by
means
of connecting grooves or the like formed in the superheater tube itself in the
contact surface thereof. '
According to the invention, the connecting groove or the like may be formed by
machining or the contact surfaces may be formed also by milling. The
connecting
groove or the like may be realized in many ways. It is, however, preferable
that
the freedom of movement of the tube elements relative to each other is
eliminated
to a reasonable extent.
The joint is formed according to a preferred embodiment of a recess, a
connecting
groove or the like, in one tube element and a protrusion in the other tube
element,
which when fitted together form a mechanical joint.
The superheater elements are preferably rectangular in cross-section and are
attached to each other by means of a groove or the like and a protrusion or
the
like, said groove and protrusion having a substantially similar profile. A
protrusion
or the like refers to an element either stationarily or movably attached to a
tube
element arranged next to a tube element provided with a groove, the protrusion
having the form of the groove.
A tube element comprises according to the invention a preferably round duct
forming a flow channel, limited by tube material in such a way that the outer
surface of the tube element is formed within a distance from the surface of
the
flow channel of two substantially parallel planar surfaces and two contact
surfaces
substantially perpendicular to said parallel planar surfaces. A joining groove
or the
like and/or a protrusion or the like substantially similar of their cross-
sectional
profile are formed on said contact surfaces.
The cross-section of the joining groove or the like in the tube element and
the
protrusion or the like is preferably a parallelogram, a semiparallelogram, or
it is
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circular, hemi-spherical, or triangular in form or a combination thereof. Even
some other forms realizing the purpose of the invention may be applied, if so
desired. The joining groove or the like of a tube element is preferably larger
of
cross-section or as large as the protrusion or the like.
By means of the present invention it is possible to achieve significant
advantages
over the prior art technique. The invention eliminates or minimizes the need
of
welding in the tube direction of the furnace superheaters, The time needed for
mart~acturing of panel structures is significantly reduced, whereby also the
time
needed for the panel structures to pass through the manufacturing process is
reduced. The operational steps of welding, such as preheating, pre-welding,
welding of both sides in tube direction, levelling of deforniations caused by
stresses and inspection of welding are replaced by an arrangement in
accordance
with the present invention.
The invention is discussed more in detail, by way of example, with reference
to
the accompanying drawings, in which
Fig. 1 schematically illustrates an example of a circulating fluidized bed
reactor
provided with a furnace superheater in accordance with the present invention;
Fig. 2 schematically illustrates a preferred embodiment of a furnace
superheater of
Fig. 1;
Fig. 3 schematically illustrates a second embodiment of a furnace superheater
of
Fig. 1;
Figs. 4-6 schematically illustrate different cross-sectional profiles of a
furnace
superheater in accordance with the present invention; and
Fig. 7 schematically illustrates yet another embodiment in accordance with the
invention to form a furnace superheater of Fig. 1.
Fig. 1 illustrates a circulating fluidized bed reactor 2, comprising a
combination of
a reaction chamber 4, a hot gas separator 6 and a return duct 8, into which
reactor
fluidizirrg gas is introduced through conduit 10 to the lower part of the
reactor
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chamber 4 or elsewhere (not shown). Solid material, such as fuel, is supplied
through a conduit 12 or through a number of conduits (not shown). Product
gases
are discharged from the hot gas separator through a conduit 14. A heat-
generating
process is maintained in the reaction chamber. This heat is recovered to the
evaporating boiler water, for example, through reaction chamber walls 16
formed
of evaporator tubes. Water to be evaporated is introduced into the evaporator
tubes
from a cylinder 18 through conduits 20 and the steam/water mixture flows back
to
the cylinder, for example, via a conduit 22. The described circulation cycle
contains a lot of generalizations and simplifications, in reality such a cycle
includes
a variety of other components. Also the hot gas separator is preferably formed
of
a evaporator or other heat exchange surfaces. The circulation cycle may
according
to the invention be of natural circulation or forced circulation type; the
invention
may be applied to both arrangements. A furnace superheater 24 is provided in
the
upper part of the reaction chamber, and steam is supplied to the furnace
superheater 24 via means 26. Steam is led from the superheater via a channel
28
to a turbine 30 and from there further back to the cylinder via a channel 32.
This
circulation cycle is also heavily simplified.
The furnace superheater 24 in the upper part of the reaction chamber 4 is
arranged
to superheat steam by utilizing heat released by the solids-gas-suspension of
the
reactor. The superheater 24 is arranged in the circulating fluidized bed
reactor in
such a way that it preferably extends at both ends outside the opposed walls
of the
reaction chamber. Thus it may, on one hand, be steadily and reliably supported
on
the ends to the reaction chamber. On the other hand, a relatively long
distance
2S between the reaction walls sets certain requirements on the panel. In
practice, a
structure, which is poorly supported easily begins to vibrate or it may bend
due to
possible thermal expansion. In other words, the superheater panel must be
sturdy
enough. This is arranged according to the present invention by supporting the
tube
elements longitudinally to each other by means of a mechanical joint. This
kind of
a mechanical joint is according to the invention achieved by providing all
surfaces
between adjacent tube elements with contoured interconnecting surfaces so that
contact surfaces facing each other fit into each other and said contact
surfaces also
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to a sufficient extent prevent movement of elements relative to each other.
The
ends of the superheaters are provided with conduits 34 to control and/or
divide the
steam flow in a desired manner between the separate tube elements of the
superheater. The furnace superheater is according to the invention formed by
joining tube elements mechanically to each other, whereby no welding in the
tube
direction is necessary. A tube panel formed in accordance with the present
invention is, however, tightly connected to conduits 34, preferably by welding
the
tube panel to these conduits, so that the demands set by steam production are
fulfilled.
The conditions in a circulating fluidized bed reactor are very erosiveo a
relatively
high temperature (e. g. 600-1200 ° C) combined with a solid particles
suspended in
the gas. In these conditions, surfaces erode very easily, especially at points
of
discontinuity. Therefore, a furnace superheater in accordance with the
invention is
preferably formed of a number of tube panels arranged one on top of the other,
which again are formed of a number of tube elements 36 joined to each other
and
the surfaces of which are arranged substantially planar by means of the method
of
forming a furnace superheater in accordance with the invention.
Fig. 1 illustrates a circulating fluidized bed reactor arranged for fuel
combustion.
A furnace superheater in accordance with the invention may, if so required, be
used also in other kind of furnaces, even for example in so called pulverized
fuel
boilers. The best characteristics of a furnace superheater in accordance with
the
present invention become apparent especially in conditions, in which the
superheater is surrounded by a gas mixture containing solid material. Thus the
circulating fluidized bed reactor may also act as a hot gas cooler, for
example, in
a grate incinerator, whereby mainly the grate arrangement deviates from the
above
described A furnace superheater in accordance with the present invention may
be
located in the furnace at different levels, although it is in Fig. 1 located
in the
upper part of the furnace.
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Fig. 2 illustrates an arrangement of a furnace superheater tube panel
according to
the invention. It shows two tube elements 362, 366 connected to each other by
means of a joint groove 364, which is formed by a recess 368 in one tube
element
362 and a protrusion 370 in another tube element. The protrosion and recess
form
together a joint, which to a sufficient extent bind the elements to each other
and
makes the superheater panel sturdy. The protrusion or recess may be arranged
in
the tube elements so that each element comprises a recess at one end and a
protrusion at the other end. Alternatively, there are two kinds of tubes:
tubes with
alternatively protnisions or recesses, whereby equal amounts of both tube
types are
required to form a tube panel. The first mentioned alternative is more
advantageous, because in that way only one kind of tube elements are needed.
This
alternative is illustrated in Fig. 3. Flow channel 372 for steam to be
superheated
is arranged inside a tube element. The tube element is formed according to the
invention so that it comprises two substantially parallel and planar surfaces
374,
376 spaced apart at a distance which is determined so that a duct or flow
channel
372 and also enough material between the surfaces and the channel 372 is
present
in the area 378 between the surfaces. The element has a mainly rectangular
cross-
sectional profile with ends 380, 382 in the contact surfaces being connected
to
joining grooves or the like in adjacent elements. Surfaces 374 and 376 extend
far
enough, so as to obtain contact surfaces with recesses and protrusions and a
sufficient material strength.
Figs. 4-6 illustrate different ways of forming contact surface required for
the joints
of the tube elements. In Fig. 4 the protrusion and the recess have one
perpendicular end surface 41 and one inclined end surface 43. In Fig. 5,
protrusions and recesses are approximately formed as circular surfaces,
whereas in
Fig. 6 they are triangular.
The method shown in Fig. 7 of joining tube elements of a furnace superheater
to
each other is very advantageous. There the opposed contact sides of two
adjacent
tube elements 70 are both provided with recesses 72, and the tube elements 70
when arranged adjacently form a passage 74 between the tubes. The tubes are
joined
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to each other by fitting a bar or the like 76 imitating the form of the
passage 74 into
it. This bar 76 corresponds to the protrusion or the like, but is not
stationarily
connected with the tube elements. This bar or the like does not have to extend
continuously from one end of the tubes to the other, but the joint may be
formed
of several shorter pieces, which are located to the space 74 according to a
particular positioning. The form of the bar or the like is preferably elected
so that
it substantially prevents the movement of the tube elements, at least the
movement
spacing the elements apart from each other. By means of the arrangement in
Fig.
7 it is possible to release, for example, one tube element from the tube panel
by
removing the bar from both sides thereof or element 76 respectively by means
of
a suitable tool. This is a very advantageous feature especially in connection
with
a circulating fluidized bed reactor, as a possible repairing of a furnace
superheater
by replacing a tube with another may be realized very simply and quickly.
The accompanying Fig. 1 illustrates a circulating fluidized bed reactor
operating at
atmospheric conditions, but a furnace superheater in accordance with the
invention
is especially suitable in combustors, which operate at pressures above
atmosphere,
for example, at 2-SO bar.
Although the accompanying drawings illustrate a number of embodiments, it is
clear that all modifications evident for a person skilled in the art are
within the
scope of invention determined by the enclosed patent claims.
